1. Field of the Invention
The invention relates to a failure diagnostic apparatus for a variable valve mechanism which detects a valve characteristic value of an internal combustion engine by using a sensor, and which adjusts the valve characteristic value based on the detected valve characteristic value, and a failure diagnostic method for the variable valve mechanism.
2. Description of the Related Art
There is a known variable valve mechanism which adjusts an intake air amount by changing a valve working angle in order to improve fuel efficiency in an internal combustion engine. In such a variable valve mechanism, there are provided an actuator for driving the mechanism and a sensor for detecting an adjustment state of the valve working angle. However, when a failure occurs in the actuator or the sensor, the valve working angle cannot be adjusted. As a result, in some cases, the internal combustion engine stops due to a decrease in the intake air amount and refuge running cannot be performed. In order to address problem, a technology is proposed in which whether a failure has occurred in an actuator or a sensor is determined, and when it is determined that a failure has occurred, a valve working angle is changed such that the refuge running can be performed. For example, Japanese Patent Laid-Open Publication No. 2000-314329, i.e., JP-A-2000-314329 (refer to pages 6 to 7, and FIG. 11) discloses a technology. In this technology, when there is an abnormality in a sensor, a valve working angle is estimated based on an operating state of an internal combustion engine, and an actuator of a variable valve mechanism is driven such that the valve working angle becomes a target value for the case where a failure has occurred.
In the above-mentioned technology, the valve working angle is estimated based on the operating state of the internal combustion engine. Therefore, when a failure in the sensor is detected, the internal combustion engine needs to be operating actually. If the operation of the internal combustion engine has been stopped or the internal combustion engine is in a transition state when it is determined that a failure has occurred in the sensor, the valve working angle cannot be changed to the target value. Accordingly, there is a high possibility that it will become difficult to start the internal combustion engine or to operate the internal combustion engine continuously and stably, making it impossible to perform the refuge running.
Further, if a failure has occurred in the actuator instead of in the sensor and the valve working angle cannot be changed, it becomes impossible to increase an output of the engine by increasing the valve working angle and to perform the refuge running. Also, a force supplied from a valve side is applied such that the valve working angle is decreased, depending on a structure of the variable valve mechanism. However, when a failure has occurred in the actuator, it becomes impossible to resist this force. Therefore, the internal combustion engine is stopped, afterwhich it is impossible to start the internal combustion engine and perform the refuge running.